In the construction of rigid vinyl window assemblies, for example, of the type disclosed in U.S. Pat. Nos. 4,259,813, 4,428,156, 4,555,868, and 4,580,366, the window and sash frames are formed of sections of extruded rigid vinyl or similar plastics material. It has been found desirable to form each of the frames with fusion-welded mitered corner joints in place of mechanical fasteners in order to provide a fluid-tight frame with substantial rigidity and to avoid the possibility of the fasteners becoming loose after period of time or use.
One of the problems encountered with welded mitered corner joints on the window and sash frames, is the creation of flash at the corner joints. The external flash which projects outwardly from the frame corners may be quickly and efficiently removed with the aid of a trimming machine. However, the internal flash requires hand trimming which adds significantly to the time and cost for producing a window frame with welded mitered corner joints. It is also desirable for the sill member of the window frame to have a slope for water drainage but without adding on or attaching another sill section such as disclosed in above mentioned U.S. Pat. No. 4,580,366. The additional sill section not only adds costs to the frame but also increases the overall height of the sill and thereby reduces the height of the viewing area through the glass unit.
It has also been found desirable in new construction for a vinyl window frame to incorporate an integral nailing flange through which nails may be driven without requiring a prepunched or drilled hole. Furthermore, when the window assembly is combined with another window assembly, it is frequently desirable to remove the opposing nailing flanges so that the window assemblies may be installed in closely spaced relation. One form of integrally formed or extruded nailing flange is disclosed in above mentioned U.S. Pat. No. 4,259,813.